Glass articles such as lenses, prisms, mirrors, CRT screens and the like may be found in any of a variety of locations including homes, offices and factories. Glass surfaces on such articles may be flat or contoured. Some glass articles include surfaces used with optical or mechanical components that require the surface to be optically clear with no visible defects or imperfections such as scratches and/or microscopic pits and the like. Contoured or curved glass surfaces such as those on CRT screens, for example, are characterized in part by the radius of the contoured surface formed in the glass forming process. During the forming process, defects such as mold lines, rough surfaces, small points and other small imperfections may be present on the outer surface of the glass. These types of imperfections, however small, can detrimentally effect the optical clarity of the glass or its desired surface flatness, and known processes have been widely used to remove these imperfections. These processes typically comprise abrasive finishing processes that can be categorized as grinding, lapping, fining and polishing.
The grinding process may be used to refine a curved contour or improve the flatness of a glass surface and to remove casting defects. This is accomplished by a rough grinding process on the glass surface using an abrasive tool. The grinding tool typically contains superabrasive particles such as diamond, tungsten carbide, cubic boron nitride or combinations thereof The grinding process is used to remove large amounts of glass quickly while leaving as fine a scratch pattern as the tooling and abrasive materials will allow. Scratches and other surface imperfections left from the rough grinding process are then removed during subsequent processing steps known as xe2x80x9cfiningxe2x80x9d and xe2x80x9cpolishingxe2x80x9d. One problem associated with the rough grinding process is that it can impart coarse scratches and conchoidal fractures within the ground glass surface as well as cause fractures beneath the surface. These surface and subsurface imperfections can extend a significant distance beneath the ground surface. As a result of these residual imperfections, the resulting glass surface after grinding is typically not smooth enough for a direct polishing step.
As an alternative to the above described rough grinding process, so called xe2x80x9cductile grindingxe2x80x9d has been developed for glass and has shown some promise for the grinding of glass and other materials such as ceramics, for example. The ductile grinding process strives to carefully control the amount of grinding force exerted on a glass surface to thereby perform the grinding step without the resulting fractures normally seen during the rough grinding step. In one mode of operation, a high speed ductile grinding process has been accomplished using abrasive grinding wheels mounted on high speed machinery with the grinding wheels comprised of very fine abrasive grit. During the process, the grinding wheel abrades the glass surface with careful control of the amount of force exerted on the glass surface by the abrasive grit within the wheels. The amount of force that the glass can tolerate without fracture is known to be influenced by the type of glass being used, the shape of the individual particles of abrasive grit, and the grinding environment. Proper control of the force exerted by the grinding wheel has been maintained by the careful positioning of the grinding wheel against the glass surface and by limiting the force applied by the grinding wheel against the surface. Other modes of ductile grinding are also known, typically requiring a flexible abrasive article that is operated at low speeds with related material removal rates that are also very low.
Ductile grinding may be desirable because it tends to avoid much of the damage that has characterized the rough grinding process, particularly the scratches and fractures that extend beneath the surface of the glass. Although ductile grinding has been effective in avoiding certain surface defects, the ductile grinding process has inherently been inefficient and/or too costly when compared to other rough grinding processes. For example, the use of the aforementioned abrasive wheels requires high speed machinery that can fail after a certain number of hours of operation, thereby necessitating the costly replacement of substantial pieces of machinery. Other ductile grinding modes utilizing flexible abrasive articles (e.g., endless belts or flexible disks) have been inefficient because the grinding process is slow with a very low material removal rate.
Subsequent to the rough grinding step, glass fining and polishing may be accomplished with loose abrasive slurry comprising a plurality of abrasive particles dispersed in a liquid medium (e.g., water). In these known finishing processes, a slurry is pumped between the glass surface and a lap pad typically made of rubber, foam, polymeric material or the like. Both the glass work piece and the lap pad may be rotated relative to each other, and this grinding process may comprise one or more steps with each step generating a progressively finer surface finish on the glass. The fining process has been required to remove the above described surface and subsurface imperfections created by the rough grinding process.
It is desirable to further refine the ductile grinding process to allow high speed grinding with relatively low cost abrasives while avoiding machine failure. It is also desirable to provide a glass finishing process that incorporates ductile grinding for high speed operations with reduced machinery failure and with a surface finish that may quickly be further processed during the xe2x80x9cpolishingxe2x80x9d step. It is also desirable to provide a process for grinding glass that is efficient and economical.
The present invention is directed to a method for grinding glass surfaces. In one aspect of the invention, a method of grinding a glass workpiece is provided comprising the steps of:
contacting a grinding layer of a flexible abrasive article with the surface of a glass workpiece, the grinding layer comprising abrasive grit dispersed in a bonding matrix, the matrix attached to a flexible backing; and
moving the grinding layer of the flexible abrasive article and the surface of the glass workpiece relative to one another at a velocity of at least about 16.5 meters per second to provide a final surface roughness Ra less than about 0.030 micrometer.
Preferably, the grinding process of the invention is performed with a liquid coolant and/or lubricant between the surface of the workpiece and the grinding layer of the abrasive article. One suitable liquid is a mixture of 20% by weight glycerol in water. The flexible abrasive article is preferably in the form of an endless belt, a web or an abrasive pad, and the grinding layer of the flexible abrasive article preferably includes composites comprised of abrasive grit in a bonding matrix with the bonding matrix affixed or adhered to a flexible backing. The composites (further described herein) are preferably in the form of truncated pyramids, but may be provided in any of a variety of configurations. The abrasive grit may be any of a variety of materials but is typically a superabrasive material and preferably comprises either single diamonds or a plurality of diamond bead abrasive particles. Useful binders preferably comprise filler in an amount from about 40 to about 60 percent by weight of the grinding layer. Diamond bead abrasive particles preferably comprise about 6% to 65% by volume diamond particles having an effective diameter of 25 microns or less with the diamond particles distributed throughout about 35% to 94% by volume of a microporous, nonfused, metal oxide matrix. Following the grinding step, the surface of the glass workpiece may be polished to provide an optically clear surface.
In another aspect, the invention provides a method of grinding a glass workpiece comprising the steps of:
contacting a grinding layer of a flexible abrasive article with the surface of a glass workpiece, the grinding layer comprising abrasive grit dispersed in a bonding matrix, the matrix attached to a flexible backing; and
moving the grinding layer of the flexible abrasive article and the surface of the glass workpiece relative to one another to provide a cut rate greater than about 7 micrometers per minute and a final surface roughness Ra less than about 0.030 micrometers.
The use of certain terminology used herein will to be understood to have definitions consistent with the following:
xe2x80x9cPrecisely shapedxe2x80x9d refers to abrasive composites formed by curing a binder precursor within a cavity of a production tool. Precisely shaped abrasive composites have a three dimensional shape defined by relatively smooth-surfaced sides that may be bounded by and joined at distinct edges having distinct edge lengths with endpoints defined by the intersections of the various sides. However, the abrasive composites may be formed as any of a variety of shapes with or without the aforementioned edges. Exemplary shapes include cylinders, domes, pyramids, rectangles, truncated pyramids, prisms, cubes, cones, truncated cones and the like. Typically, the abrasive composites will have a cross-section in the form of a triangle, square, circle, rectangle, hexagon, octagon, or the like.
The abrasive composites may also be irregularly shaped in that the sides or boundaries of the composites are slumped and not precise. An irregularly shaped abrasive composite may resemble conventional shapes such as the aforementioned cylinders, domes, pyramids, rectangles, truncated pyramids, prisms, cubes, cones, truncated cones and the like. However, the irregularly shaped abrasive composite may appear to be somewhat deformed or not fully formed. Alternatively, an irregularly shaped abrasive composite may have a three dimensional form in that it has a height, thickness and a base dimension, while not bearing a resemblance to any of the foregoing conventional shapes. In forming an irregularly shaped abrasive composite, the abrasive slurry may be first formed into a desired shape and/or pattern. Once the abrasive slurry is formed, the binder precursor in the abrasive slurry is typically cured or solidified. There is generally a time gap between forming the shape and curing the binder precursor. During this time gap, the abrasive slurry is still capable of flowing. Abrasive composites can also vary in size, pitch, or shape in a single abrasive article, as described in WO 95/07797, published Mar. 23, 1995 and WO 95/22436, published Aug. 24, 1995.
xe2x80x9cTexture,xe2x80x9d as used herein, refers to a grinding layer on an abrasive article having any of the aforementioned three dimensional composites, whether the individual three dimensional composites are precisely shaped, irregularly shaped, or comprise a combination of precisely shaped and irregularly shaped composites. The texture may be formed from a plurality of abrasive composites which all have substantially the same shape. Similarly, the texture may be in a random pattern where the shapes of the abrasive composites differ from one to another in the same abrasive article.
xe2x80x9cRaxe2x80x9d as used herein refers to a surface roughness measurement made with, for example, a Tencor P2 Long Scan Profiler (KLA Tencor; Mountain View, Calif.) with a 0.2 micrometer stylus and a 40 milligram stylus force. The scan speed is 0.02 millimeters/second and the scan sampling length is 0.25 millimeters with an evaluation length of 1.25 milimeters. The cutoff wavelength is 0.25 millimeters. Generally, the lower the Ra value, the smoother the finish.
xe2x80x9cConchoidal fracturexe2x80x9d means a fracture in a glass surface having a shape roughly resembling that of a clam shell half or overlapping portions thereof.
xe2x80x9cDuctilexe2x80x9d as used in reference to the grinding process refers to removing material smoothly with an abrasive implement resulting in a surface containing fine striations.
In still another aspect of the invention, a method is provided for the grinding of a workpiece comprising the steps of:
contacting a grinding layer of a flexible abrasive article with the surface of the workpiece, the grinding layer comprising abrasive grit dispersed in a bonding matrix, the matrix attached to a flexible backing; and
moving the grinding layer of the flexible abrasive article and the surface of the workpiece relative to one another at a velocity of at least about 16.5 meters per second to provide a final surface roughness Ra less than about 0.030 micrometer.
These and other aspects of the invention will be more fully appreciated by those skilled in the art upon further consideration of the remainder of the disclosure including the Detailed Description of the Preferred Embodiments and the appended claims.